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An even number is defined as any number that has 2 as a factor. For example, 2, 4, 6, 8 and 10 are all even numbers. Any number without 2 as a factor is odd, like 3, 5, 7 and 9. Because 2 is a factor of all even numbers, it can be factored out. The sum of x+y then looks like this: 2*(x/2+y/2). No matter what the numbers inside the brackets are, after being multiplied by 2, the final result will then have 2 as a factor, which makes it even.